Reversing circulator in a DHW zone (steam system)

Jim Franklin

I'll definitely be repiping the DHW loop, but the two big reasons I'd like to convert to FHW are the costs of running the existing steam (mostly caused by the hour or so it takes for steam to start heating the house) and because I have a new 1000 SF 2nd story to pipe for heat. The other reasons are smaller (multiple zones, no bulky rads, get to play with cool technology) but not insubstantial.

The slow/expensive steam can be mostly fixed by repiping the header. That's a day's work plus materials so my seat of the pants guess is $800-$1000, including a few necessary main & radiator vents. For not much more than that I can pipe & zone the entire house myself.

I'm all ears on ways to keep the steam and still get reasonable response times and efficiency. If I worked from home and used the whole house, the steam would be much more attractive. But using only a few rooms at certain parts of the day seems to call for a more modular heating approach.

jim

Jim Franklin

DHW zone steams

My steam system has a DHW zone with indirect tank. The installer used a tapping about halfway up the boiler and pumped away. The zone occasionally vapor locks and I need to shut things down and let it cool. Last night it wouldn't stop vapor locking so I decided to switch the circulator direction. It was then I noticed the TWO check valves. Can I loosen the thumbscrews on top to eliminate the checking? Getting in there to un-solder would make Houdini blanch. I plan on re-piping sometime this summer so it'd be temporary.

Barring that, is there any harm in raising the water level enough to stop the vapor locking? I won't need steam until October.

thanks,
jim

Al Letellier_9

vapor lock

You can raise the checks,but the flow will be reversed and you will probably get some rattle from the checks. Better to find out why the zone is "vapor locking" and fix the problem, not the syptoms. What you propose is only a temp fix. I have many of these loops off steam boilers, one of them I installed 18 years ago and it's still working. Never been touched....you have a problem you haven't found yet.

Jim Franklin

Where do you place teh circ & what direction do you pump?

I assume it's vapor locking because the control for the DHW zone has no high water temp cutoff; it keeps firing the boiler until the aquastat in the tank stops the call, so occasionally it pulls steam into the loop. I know this is wrong and plan to correct it by swinging the house over to FHW.

Apparently it was just severely airbound today. I lifted the checks and had to close the return valve and bleed it out the bottom of the indirect coil. I didn't hear any air but after I got hot water out of the coil I buttoned it up and it started pushing a little water. A few on/off cycles of the return valve purged the rest of the air.

jim

Larry C_10

DHW is getting steam bound, so you are converting to FHW?

DHW is getting steam bound, so you are converting to FHW? Did I read the previous post correctly?

Quote "I assume it's vapor locking because the control for the DHW zone has no high water temp cutoff; it keeps firing the boiler until the aquastat in the tank stops the call, so occasionally it pulls steam into the loop. I know this is wrong and plan to correct it by swinging the house over to FHW."

Wouldn't it be easier and cheaper to replace the Aquastat with a correct one?

Steamhead (in transit)

Jim, it would be a waste of time and money

to tear out the steam and put in hot-water.

I think part of the problem is that the indirect loop circ is taking water from high in the boiler. There is little if any static pressure at this point, so the water can easily flash to steam. Compounding the problem is a lack of a high-limit control that shuts down the burner at 180 degrees or so, if only the indirect is calling for heat.

The circ should take water from near the bottom of the boiler and pump away from the boiler toward the tank. This will prevent vapor locking. I doubt the check valves are needed here.

An aquastat is then added to the boiler that stops the burner before the water can boil. The burner circuit is wired to bypass this aquastat if the boiler has to make steam.

Try it this way and watch how well it works.

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Steamhead (in transit)

The first thing I'd do

is vent the mains properly. You want to have the steam reach the ends of the mains about a minute after it starts leaving the boiler. Measure the length and diameter of your steam mains and we'll tell you what you need.

Then, see how long it takes for the boiler to start producing steam. A modern boiler should do this in 10 minutes from a cold start. If it takes longer, there's a problem with the boiler. This is something you ought to have a pro look at.

Repiping the header will help if the current piping is wrong. I'd oversize the header a bit, for example if the specs called for a 2-inch or 2-1/2-inch header, I'd use 3-inch. This would produce even drier steam. In the attached photo, which is on a Kriebel Vapor system, we ran the output from the 3-inch drop header (size called for in I&O manual) into the input to the original 5-inch header. This produces some bone-dry steam and the system heats up lightning-quick!

You can zone a steam system room-by-room, by using thermostatic radiator valves. Since you mentioned "radiator vents" you probably have a one-pipe system. In this case, the TRVs would replace the existing radiator air vents.

If you wanted to put hot-water on the second floor, you could use a separate boiler for this area, and pipe the present indirect to it. This solution would let you play with the new stuff ;-) It would also mean that if one boiler broke down, you'd still have some heat.

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